Using PGraphics for Off‑Screen Rendering in Processing

Useful answer

PGraphics creates an off‑screen buffer that you can draw to once and reuse many times. This lets you layer effects, apply post‑processing, or avoid redrawing static content each frame without re‑executing the main draw() loop.

How it works

First allocate a PGraphics object with createGraphics(width, height, renderer). The renderer must be P2D or P3D; the default JAVA2D ignores off‑screen drawing. Inside a beginDraw() / endDraw() pair you issue any Processing drawing commands just as you would on the main canvas. After endDraw() the buffer holds the rasterised result, which you can display with image(pg, x, y) or use as a texture.

Worked example

PGraphics pg;
void setup() {
  
size(400, 400, P2D);               // main canvas uses P2D renderer
  
pg = createGraphics(width, height, P2D); // off‑screen buffer
  
pg.beginDraw();
  
pg.background(200);               // light gray
  
pg.fill(0);                       // black
  
pg.ellipse(200, 200, 100, 100);   // circle in the centre
  
pg.endDraw();
}
void draw() {
  
background(30);                  // dark background each frame
  
image(pg, 0, 0);                  // draw the buffered circle
}

The circle appears on a dark background. Changing the circle’s position inside the beginDraw()/endDraw() block updates the displayed image without redrawing the gray background each frame.

Limits and common mistakes

• Memory use – the buffer occupies roughly width × height × 4 bytes (RGBA). Large buffers can exceed GPU memory and cause slowdowns or OutOfMemoryError at startup.
• Renderer dependence – off‑screen drawing only works with P2D or P3D. Using the default JAVA2D renderer (e.g., size(400,400) without a renderer) will produce no visible result from the PGraphics buffer.
• Missing begin/end – issuing drawing commands outside a beginDraw()/endDraw() pair affects the main canvas, not the buffer, and the off‑screen image stays blank.
• Re‑creating each frame – calling createGraphics inside draw() defeats the performance benefit and repeatedly allocates memory.

Practical verification

1. Run the example code in Processing 4.x. You should see a gray circle on a dark background.
2. Modify the sketch to use JAVA2D (remove P2D from both size() and createGraphics()). The circle will disappear, confirming the renderer requirement.
3. Check memory usage with Processing’s built‑in profiler or an external tool; observe that memory grows linearly with the buffer’s pixel count.